Gold plating



United States Patent GOLD PLATING Lawrence Greenspan, New York, N.Y., assignor to Engelhard Industries, Inc, Newark, N.J., a corporation of Delaware No Drawing. Filed Feb. 23, 1966, Ser. No. 529,262

US. Cl. 20443 6 Claims Int. Cl. (12% 32; C23]: 5/28 The present invention relates to gold electroplating baths and more particularly to a gold plating solution for the electrodeposition of heavy, smooth ductile and pure gold deposits.

Gold plating baths presently in commercial use are generally of two types: (1) alkaline baths containing gold in the form of potassium gold cyanide and substantial concentrations of uncombined potassium or sodium cyanide, and (2) so-called acid gold baths, operating in a pH range of about 3 to 6.5, which contain gold in the form of a cyanide complex and salts of organic acids such as citrates, acetates, lactates, tartrates, gluconates, etc. In addition many of these baths contain complex organic chelating agents such as e-thylenediamine tetraacetic acid or other aminopolycarboxylic acids. Yet others contain organic complexing agents such as ethylenediamine, diethylenetriamine and other complex organic amines, or organic compounds such as triglycollamic acid. The alkaline baths containing free cyanide are disadvantageous in that carbonates accumulate and decomposition products due to oxidation and hydrolysis of the free cyanide build up and adversely affect the quality of the electrodeposited gold by occlusion of small quantities of these impurities in the gold plate. On the other hand, baths operating in the acid range, containing complex organic materials such as mentioned above are subject to severe oxidizing conditions at the surface of the insoluble anodes that are of necessity used in these solutions, with the result that complex organic decomposition products are formed and by being occluded in small amounts in the electrodeposited gold seriously impair the quality and physical characteristics of the deposit. The adverse effects may manifest themselves as brittleness, off-color, dark deposits, having poor conductivity and poor solderability.

The present invention is intended to overcome such difficulties and disadvantages, it being among the objects hereof to provide a bath composition which permits obtaining heavy smooth deposits of gold free from organic decomposition products.

Another object of the invention is the provision of a bath composition which is capable of obtaining deposits that are not only heavy and smooth but also are of a high degree of purity and highly ductile.

It is another object of the invention to provide a bath of good stability and simple to control and operate.

In the practice of the invention, it has been found that, by the use of gold plating baths containing gold in the form of an alkali metal complex such as potassium or sodium gold cyanide, pyrophosphates of the alkali metals including ammonium as conducting salts, and a tri-valent arsenic compound, with the pH of the solution being adjusted in range of 5 to 8 by the addition of pyrophosphoric acid or Superphosphoric acid, the disadvantages inherent in baths containing complex organic materials are significantly eliminated and it is possible to obtain gold deposits of exceptional purity and good ductility.

It has been found that best results for providing heavy durable deposits may be obtained at appreciably low temperatures by the introduction into the solution of very small amounts of a tri-valent arsenic compound such as arsenic trioxide (AS203) and arsenious acid 3,423,295 Patented Jan. 21, 1969 s a s) with or without salts of organic acids, such as ammonium citrate, so long as pyrophosphates of the alkali metals are employed. The arsenic compounds employed are critically in the trivalent state, since elemental arsenic and other multivalent com-pounds of arsenic, such as pentavalent arsenic compounds, e.g., arsenic pentoxide have little or no effect in providing gold deposits of good quality and of substantial thickness and smoothness. The advantages are particularly evident at relatively low temperatures in the range of ll5-120 F. This is a highly desirable factor particularly in the plating of circuit boards the plastic portion of which may not tolerate temperature above F. The presence of tri-valent arsenic in minute amounts appears to act catalytically or by exercising a depolarizing effect in extending the limiting current density and improving the smoothness and color of the electrodeposited gold. As little as 0.2 milligram per liter is effective as shown by Hull cell plating test, additional improvement occurring up to 2 mg. per liter and very little is to be gained by going above 10 mg. per liter. Careful analyses of the deposits show no trace of arsenic. It has been known to incorporate arsenic in other type gold cyanide solutions but in all these cases arsenic is used in much greater quantities and is invariably co-deposited with the gold, impairing the physical characteristics of the deposit, particularly the ductility.

This invention consists in providing a bath composition using pyrophosphates as the essential conducting salts. Pyrophosphates, by virtue of their complexing ability, have been found to be considerably advantageous over phosphate salts. Although it is known that the pyrophosphates revert slowly to orthophosphates, it has been observed that the gold plating results are not adversely affected probably because intermediate degradation products still possess complexing ability and the beneficial effects of pyrophosphates are evident even when relatively small amounts are present. Pyrophosphates appear also to have a stabilizing effect on baths containing organic acid salts such as citrates, tartrates, etc. as evidenced by the fact that the solutions remain Water clear whereas in the absence of pyrophosphates these solutions quickly discolor assuming a yellow to brown tinge indicating decomposition of the organic acid of the salt. Also in the practice of this invention, fresh pyrophosphate is continuously added to the bath along with replenishment of the gold cyanide salt. Another source of pyrophosphate ion replenishment is occasioned by the fact that the pH of the bath will rise with use, and periodic additions of Superphosphoric acid are made to maintain the pH of the bath between about 5 and 8. Reversion to orthophosphate becomes less as the pH is increased from the acid side to neutral and becomes from a practical standpoint a negligible factor at pH 7 or above. Superphosphoric acid is a commercial product containing the highest percentage of pyrophosphoric acid (about 42%) of any of the phosphoric acids commercially available. Thus, adjustment of the bath pH with this acid effects replenishment of pyrophosphate ion to the solution to assure its presence in the bath in appreciable amount to counteract the tendency to reversion to orthophosphate. However, with the pyrophosphate present in adequate amount, the pH may be adjusted with other suitable acids, phosphoric acid, polyphosphoric acid or acid salts such as ammonium citrate, monobasic potassium phosphate; boric acid, formic acid and tartaric acid. The concentration 6r potassium pyrophosphate may vary between about 10 grams per liter up to saturation without significantly "ice affecting the quality of the gold electrodeposit although the preferred range is 75 to 200 grams per liter. In practice, the saturation point is seldom reached because the drag-out increases with increased concentration of pyrophosphate and obviates against a point of saturation being reached.

In general, the electrolyte solution contemplated is an electrolyte solution for use in the electrodeposition of gold comprising essentially from about 4 to 50 grams of an alkali metal gold cyanide per liter, about 10 to 200 grams of an alkali metal pyrophosphate, 10 to 100 grams of a Superphosphoric acid per liter, or an acid or salt above-mentioned, and from 0.2 to 10 milligrams of a tri-va'lent arsenic compound such as arsenic trioxide, the pH of the solution being adjusted to a value in the range of from and 8.

In addition to the above, the solution may contain from 1 to 110 grams of the salt of an organic acid such as ammonium, potassium or sodium citrate.

The term an alkali metal gold cyanide includes potassium gold cyanide and sodium gold cyanide.

The term a potassium pyrophosphate includes potassium pyrophosphate and tetrapotassium pyrophosphate.

The term a Superphosphoric acid includes an acid containing pyrophosphoric acid as a major ingredient together with any other of the phosphoric acids.

Example 1 An electrolyte was prepared as follows:

4 grams potassium gold cyanide grams potassium pyrophosphate 10 grams Superphosphoric acid 0.2 milligram AS203 Water to make one liter.

The pH is adjusted to 5.5-6.0 using Superphosphoric acid to lower the pH. Potassium hydroxide, potassium pyrophosphate or ammonium hydroxide are employed to raise the pH. At a temperature of 5065 C. and current densities over a range of 3-6 amperes per square foot, with moderate to vigorous agitation smooth, ductile and yellow 24K deposits are obtained.

Example 2 50 grams potassium gold cyanide 200 grams potassium pyrophosphate 100 grams Superphosphoric acid 10 milligrams AS203 Water to make one liter.

The pH of the solution is adjusted to 6.5-7.0 using Superphosphoric acid to lower the pH. Potassium pyrophosphate, potassium hydroxide or ammonium hydroxide are employed to raise the pH. At temperatures of 60" C. and current densities over a range of 510 amperes per sq. ft., with moderate to vigorous agitation, smooth, ductile, yellow 24K gold deposits may be obtained up to thicknesses of 0.050.

Example 3 4 grams potassium gold cyanide 10 grams ammonium citrate 10 grams potassium pyrophosphate 10 grams Superphosphoric acid 5 milligrams As O Water to make one liter.

The pH is adjusted to 5.5-6.5 using Superphosphoric acid or ammonium citrate to lower the pH. Potassium pyrophosphate, potassium hydroxide or ammonium hydroxide are employed to raise the pH. At a temperature of -65 C. and current density over a range of 3-6 ampers per square foot, with moderate to rapid agitation, smooth, ductile and yellow gold 24K gold deposits are obtained.

Example 4 20 grams potassium gold cyanide 50 grams potassium pyrophosphate 25 grams ammonium citrate 30 grams Superphosphoric acid 10 milligrams AS203 Water to make one liter.

The pH is adjusted to 5.57.0 using Superphosphoric acid or ammonium citrate to lower the pH. Potassium pyrophosphate, potassium hydroxide or ammonium hydroxide are employed to raise the pH. At temperatures of 45 -65 and current densities of 5-10 amperes per square foot, with moderate to strong agitation, smooth, ductile, yellow 24K deposits are obtained up to heavy thicknesses of the order of 010-040".

Example 5 20 grams potassium gold cyanide grams ammonium or potassium citrate 8 milligrams AS203 Water to make one liter.

The pH is adjusted to 5.56.5 using Superphosphoric acid or ammonium citrate to lower the pH. Potassium, sodium or ammonium hydroxide are employed to raise the pH. At temperatures of 4565 C. and current densities of 5-10 amperes per square foot, with modearte to strong agitation, smooth, ductile, yellow 24K deposits are readily obtained.

What is claimed is:

1. An electrolyte solution for use in the electrodeposition of gold comprising essentially from about 4 to 50 grams of an alkali metal gold cyanide, about 10 to 200 grams of an alkali metal pyrophosphate, about 10 to grams of a pH adjusting acid, about 0.2 to 10 milligrams of a tri-valent arsenic compound and water to make one liter, the pH of the solution being adjusted to a value in the range of from 5 to 8.

2. An electrolyte solution according to claim 1, containing from about 1 to grams of the salt of an organic acid.

3. An electrolyte solution according to claim 2, wherein the conducting salt is a salt taken from the group consisting of the ammonium, sodium and potassium salts of citric acid.

4. An electrolyte solution according to claim 1, wherein the pH adjusting acid is Superphosphoric acid.

5. An electrolyte solution according to claim 1, wherein the alkali metal gold cyanide is potassium gold cyanide.

6. An electrolyte solution according to claim 1, wherein the alkali metal pyrophosphate is potassium pyrophosphate.

References Cited UNITED STATES PATENTS 744,170 11/ 1903 Darlay 204-46 XR 823,864 6/1909 Levy 204-46 XR 2,967,135 1/1961 Ostrow et al. 20446 XR 3,149,057 9/1964 Parker et al. 204-46 FOREIGN PATENTS 1,027,637 2/1953 France.

928,088 6/1963 Great Britain.

354,643 7/ 1961 Switzerland.

V JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner.

US. Cl. X.R. 20446 

